What do the different types of controls for immunofluorescence do?

The different types of controls for immunofluorescence include:

1. Positive control - the protein of interest is known to be overexpressed. If staining is absent in this control, it indicates issues with the staining protocol. 
2. Absorption control - the sample is incubated with an immune-depleted primary antibody to display its specificity. One should not expect not to observe any signal indicating specificity of the primary antibody. 
3. Omitting the primary antibody control test for non-specific binding of the secondary antibody by incubating the sample with antibody dilution buffer alone, without the primary antibody. The purpose of this is to determine whether the observed signal is a result of non-specific binding of the secondary antibody in the tissue or cell sample.
4. Isotype control - one should incubate the sample with a non-immune antibody of the same isotype and concentration as the primary antibody to check for non-specific interactions. Minimal background staining with this control is expected, particularly with monoclonal primary antibodies.
5. Omitting the secondary antibody helps differentiate between specific staining and autofluorescence, and techniques can be applied to reduce autofluorescence levels if necessary.